The present invention is directed to the production of a high durability titanium alloy useful for producing structural components for aircraft. Particularly, the present invention is directed to permitting significant weight reduction for fracture-sensitive aircraft components, particularly for high-performance aircraft, through the use of a highly fracture-resistant, high strength to density ratio titanium alloy. The high fracture resistance permits a damage-tolerant design approach. The high strength to density ratio will provide weight savings, with improved thrust to weight ratio and specific fuel consumption, with readily apparent benefits for takeoffs and landings and in aircraft flight range.
One alloy which has been widely used for structural aircraft application is a Ti-6Al-4V alloy. However, this alloy has not been completely satisfactory, particularly with respect to tensile strength. A possible replacement for the Ti-6Al-4V alloy is a titanium alloy containing (in weight percent) 6% Al, 2% Sn, 2% Zr, 2% Mo, 2% Cr and 0.23% Si (Ti-6-22-22S), which has good tensile strength. However, this alloy Ti-6-22-22S, under conventional alpha/beta processing conditions, generally has a disadvantage of low fatigue crack growth resistance. It is therefore an object of the present invention to improve the fatigue crack growth resistance of titanium alloys containing Al, Sn, Zr, Mo, Cr and Si.
Heat treatment of titanium alloys, such as annealing, solution treating and aging, may affect various properties of the alloy. Titanium alloys have a microstructure which includes a close-packed hexagonal structure (the alpha phase), which may change to a body-centered cubic structure (the beta phase) at a temperature known as the beta transition temperature or T.sub..beta.. The beta transition temperature for any given alloy is easily determined experimentally.
Some alloying agents are alpha stabilizers, and raise the beta transition temperature. Oxygen and aluminum are examples of alpha stabilizers. Other alloying agents, such as manganese, chromium, iron, molybdenum, vanadium and niobium, lower the beta transition temperature, and may result in retention of some beta phase at room temperature. Other alloying elements, such as zirconium and tin, have relatively little effect on the beta transition temperature. Some titanium alloys are two-phase alloys containing both alpha and beta phases at room temperatures. While the two-phase alloys are the most versatile of the titanium alloys, different heat treatments will be applied to different alloys for different purposes.